1. Field of the Invention
The present invention relates to a recording medium of thermally-assisted magnetic recording or microwave-assisted magnetic recording, which is used in hard disk drives (HDD) or the like, and a magnetic recording and reproducing apparatus.
This is a National Stage of International Application No. PCT/JP2013/063136 filed May 10, 2013, claiming priority based on Japanese Patent Application No. 2012-110578, filed on May 14, 2012, and Japanese Patent Application No. 2012-143084, filed on Jun. 26, 2012, the contents of which are incorporated herein by reference.
2. Description of Related Art
Recently, the demand for hard disk drives with high capacity has been increasing. However, with the existing recording methods, it is difficult to improve the recording density of hard disk drives. Thermally-assisted magnetic recording is a technique which has been actively studied and attracted attention as a recording method in next generation. In thermally-assisted magnetic recording, a magnetic head irradiates a medium with near-field light to heat a medium surface locally, thereby decreasing the coercive force of the medium to write information thereon.
In thermally-assisted magnetic recording, as a material of a magnetic layer, a high Ku material such as FePt (Ku: 7×107 erg/cm3) or CoPt (Ku: 5×107 erg/cm3) having an L10 type crystal structure is used. When the high Ku material is used as the material of the magnetic layer, KuV/kT (Ku: magnetic anisotropy constant, V: particle volume, k: Boltzmann's constant, T: temperature) increases. Therefore, the magnetic particle volume can decrease without thermal fluctuation being decreased. Due to the refinement of magnetic particles, the transition width can be narrowed in thermally-assisted magnetic recording. As a result, noise can be reduced and the signal-to-noise ratio (SNR) can be improved.
In addition, in order to obtain a thermally-assisted magnetic recording medium having high perpendicular magnetic anisotropy, it is necessary to allow an alloy having an L10 type crystal structure, which is used as a magnetic layer, to have a high (001) orientation. The orientation of the magnetic layer is controlled by an underlayer. Therefore, it is necessary to appropriately select a material of the underlayer. As a material of the underlayer of the magnetic recording medium, for example, MgO, CrN, or TiN is known in the related art.
For example, Patent Document 1 discloses a technique in which an underlayer containing MgO as a main component is prepared and an ordered alloy layer with an L10 type crystal structure formed of an FePt alloy is prepared.
In addition, Patent Document 2 discloses a magnetic recording medium, including: an underlayer that is formed of a transition metal nitride such as TiN, ZrN, HfN, or CrN; and a magnetic recording layer that is disposed on the underlayer and includes dots, which is formed of a magnetic material having an L10 structure such as FePt or CoPt, and a non-magnetic region.
In addition, as an example of a magnetic recording medium which includes plural magnetic underlayers, NPL 1 discloses a configuration in which a FePt magnetic layer exhibits a high (001) orientation by using a RuAl underlayer and a TiN underlayer. RuAl exhibits a (100) orientation due to its B2 structure. TiN has a NaCl structure as in the case of MgO and also has a lattice constant similar to that of MgO. Therefore, TiN can make a FePt magnetic layer have a (001) orientation as in the case of MgO.
In addition, another technique which has attracted attention as a next-generation recording method is microwave-assisted magnetic recording. In microwave-assisted magnetic recording, a magnetic layer of a magnetic recording medium is irradiated with microwaves to incline a magnetization direction from a magnetization easy axis and to locally reverse the magnetization of the magnetic layer, thereby recording magnetic information.